This invention relates to an efficient method of operating adsorption beds and to beds designed to exploit the method. More particularly, it relates to a method of operating zeolite, silica gel, alumina, activated carbon, thermally regenerable ion exchange resin and other such beds of the type employed to remove impurities from fluid streams such as natural gas, air, or water.
Adsorbents are widely used for the purification of fluid mixtures. The adsorbent material or materials, typically in particulate form, are contained in a vessel which provides means for passing fluid along a flow path through the interstices among the particles in the bed. A fluid feed stream containing a dilute species (adsorbate) to be removed, typically at concentrations no greater than about 15 percent, is introduced into the bed and passed along the flow path in an adsorption stage. Inside the bed an adsorption wave or front forms which passes along the flow path from a point adjacent the bed entrance in the same direction as the fluid flow, but at a much slower rate.
This adsorption front is the bed region wherein changes in adsorbent loading and adsorbate content in the fluid phase occur. The front's upstream side is bounded by a bed region characterized by adsorbent loadings, adsorbate to fluid feed mole ratios, and temperatures characteristic of equilibrium between the adsorbent material and the feed. On its downstream side, the adsorption front is bounded by a bed region having properties characteristic of equilibrium between the adsorbent material and the substantially adsorbate-free fluid product. Front boundaries are generally not well defined but rather comprise regions which asymptotically approach equilibrium. As the downstream boundary of the adsorption front approaches the bed exit, the concentration of adsorbate in the product begins to rise. When the concentration of the adsorbate in the product at the exit exceeds some predetermined specification, adsorption is discontinued and the bed is regenerated.
In the regeneration stage, a regenerant comprising a hot fluid is passed along the flow path in a co-current, or more commonly, a countercurrent direction. The high temperature of the regenerant produces a desorption front in the bed which drives adsorbate off the surface of the adsorbent material and into the flowing regenerant stream. This process continues until the bed is substantially adsorbate-free, typically as indicated by the emergence of hot regenerant fluid at the bed exit. The hot, adsorbate-free bed is then either cooled or utilized for adsorption service while still hot. The introduction of a coolant produces a thermal front which takes heat from the bed.
Adsorption bed systems of the type described above are known as "thermal swing" systems because they are regenerated with heat. They have been widely utilized in various industries. For example, in preparing air for use in pneumatic systems, water vapor is often first removed from the air using activated alumina or a zeolite. Natural gas is similarly treated before it is liquified or delivered to a pipeline. Activated carbon is used to remove trace quantities of organic vapors from air in solvent recovery operations. Similarly, carbon dioxide, mercury, oxides of nitrogen and sulfur, and hydrogen sulfide may be removed from air or exhausts on zeolite molecular sieves or on activated carbon. Recently, thermally regenerable ion exchange resins have been developed which, for example, can remove salts from water (Dabby et al., Recent Experience With A New Thermally Regenerable Deionization System, 37th Annual Int. Water Conf., Pittsburgh, Pa. 10/26-28, 1976).
Industry has tried various approaches to increasing the efficiency of operation of adsorption beds, and in some instances, improvements in the utilization of energy has been the goal. The technical literature primarily focuses on the adsorption stage of the cycle, but regeneration has also been addressed. For example, W. A. Johnston in Chemical Engineering (Nov. 27, 1972 p. 87-92) has noted that the total heat input required for a regeneration is equal to the sum of the sensible heat adsorbed by the vessel housing, the adsorbent material itself, and the mass of retained fluid in the bed, plus the latent heat of desorption required to drive adsorbate from the adsorbent material.
In the June 11, July 9, and Aug. 6 (1973) issues of Chemical Engineering, George M. Lukchis presents an analysis of the thermodynamics of bed chemistry and suggests a model of adsorption which may be used to analyze the performance of commercial units. This permits the designer of such equipment to estimate when adsorption must be stopped to prevent breakthrough, i.e., to prevent contamination of collected product with adsorbate-containing feed. It is suggested that heat utilization can be improved by external or even internal insulation which eliminate heat loss. At page 88 of the August 6 article, it is noted that cooling and heating fronts which pass through such beds are not infinitely short and that the observed spreading of fronts precludes stopping the purged gas flow as the front reaches the effluent end of the bed. In The Canadian Journal of Chemical Engineering (Volume 53, April, 1975, pages 234) D. Basmadjian in an article entitled, On the Possibility of Omitting the Cooling Step in Thermal Gas Adsorption Cycles suggests that since the heat removed from the bed after regeneration often cannot be utilized in any significant way, it may often be advantageous to entirely omit the cooling stage. The conditions under which beds can be operated in this manner are disclosed.
Other publications directed to this general topic include U.S. Pat. Nos. 3,808,773 (Reyhing et al.), 3,359,706 (Zankey), 3,738,084 (Simonet et al.), and 4,012,206 (Macriss et al.) In the Simonet patent, a bed layered with different adsorbents is used, and thermal input is minimized by reuse of heat. In the Macriss et al. patent, a method of operating an adsorbent bed is disclosed which involves the use of sensible heat remaining in the bed after desorbing gaseous oxide contaminants to aid in the removal of adsorbed water vapor contained downstream of the hot zone.